The present invention is directed to an apparatus for providing constant hydraulic output pressure from a pump, more particularly, to a pressure compensation means for a piston driven hydraulic pump.
1. Field of the Invention
The use of hydraulic pumps, in particular, piston driven or rotary hydraulic pumps, to provide high pressure water is well known in the art. The hydraulic piston pumps may be driven by pneumatic pressure, internal combustion engines, electric motors, or other means. Pneumatic hydraulic pumps are often capable of developing output pressures in excess of thirty times the pneumatic pressure supplied. Thus, a hydraulic pump having a 100 psi air pressure supply may be capable of developing a hydraulic pressure of 3,000 psi or greater. However, the design of a piston driven pump includes a known problem in that the pump does not develop any significant pressure at the piston top dead point (TDP) and bottom dead point (BDP). Thus, the hydraulic pump is not working during the entire piston travel cycle.
Further, the piston hydraulic pump also suffers from a problem known as hydraulic shock when the piston face comes into contact with the water after reaching each dead point. This hydraulic shock can result in an excessive wear to the hydraulic pump and any connecting lines to the pump thereby increasing the safety risk for the pump, operator and any downstream systems.
Prior art includes various mechanisms for eliminating hydraulic shock and for regulating pump output pressures to provide for a relatively constant pressure supply. These systems include the Hydrophor water supply systems which are commonly utilized in maritime vehicles. In the Hydrophor system, a water pump fills a closed container approximately two-thirds full of water and automatically switches off. The top end of the container is connected to a compressed air supply which maintains air pressure at a set level in the container. As the hydraulic pump begins filling the container with water, the air at the top of the container is compressed. When the amount of water in the container decreases, the hydraulic pressure within the container also decreases. The compressed air within the container forces the water downwardly, thereby compensating for the loss in hydraulic pressure from the pump. Thus, the Hydrophor system utilizing compressed air and hydraulic pressure maintains a water pressure in the range of 30-80 psi.
Another known system for maintaining relatively constant output pressure is through use of a small vessel having two chambers separated by a flexible diaphragm. One chamber is filled with compressed air while the other chamber is filled with the working pressurized liquid. As in the Hydrophor system, the diaphragm is displaced by pressurized water, thereby pressurizing the air in the other chamber. As the hydraulic pressure decreases, the air pressure deforms the diaphragm into the water chamber, partially compensating for the loss in hydraulic pressure.
The above systems, however, are not suitable in high pressure hydraulic applications. In the Hydrophor system, there is direct contact between the air and the water, which absorbs the compressed air. As the water pressure increases, the absorption of air within the water is greater. To compensate for this increased absorption of air, the air pressure itself must be increased. This requires a container having a thicker wall to compensate for the increased pressure.
The second mechanism, which utilizes a diaphragm, does not have the problem of air absorption because there is no air/water interface. However, this mechanism is unsuitable for use at high hydraulic pressures. The air chamber must be filled with an air pressure almost equal to the working water pressure. This requires higher air pressure which increases energy consumption and places significant material requirements on the diaphragm itself. The diaphragm must be elastic and thin to deform sufficiently to permit the compressed air to compensate for the drop in hydraulic pressure, but at the same time, be strong enough to withstand the high pressures. Further, this second type of mechanism has limited diaphragm deformation, thus decreasing the ability of the air chamber to compensate for the drop in hydraulic pressure.
Thus, there exists a need for a high pressure output compensation system, utilizing relatively low air pressure, which is capable of maintaining relatively constant high hydraulic pressures from a piston-driven pump.